Effect of displacement loading rates on mode-I fracture toughness of fiber glass-epoxy composite laminates

Size-effect method is used in determining mode-I fracture characteristics of woven fiber glass/ epoxy composite laminates for varying loading rates. Tensile testing of geometrically similar single-edge notch (SEN) specimens with three widths 30 mm, 40 mm and 50 mm is carried out for four different displacement loading rates namely 1, 10, 100, 500 mm/min. For each width D and loading rate, the crack-length a is varied as 0.125D, 0.25D, 0.375D and 0.5D. A total of 186 specimens are tested. The peak stresses sigma(Nu) the initial Youngs Modulus E-xx and the shear modulus G(xy) are calculated. The fracture toughness K-IC, critical strain energy release rate G(f) and material characteristic length C-f are calculated using linear regression analysis of non-linear fracture mechanics equations. The values of G(f) and C-f first decrease and then increase monotonically as loading rate changes from 1 to 500 mm/min. This behavior was explained using Bazant size-effect equation relating nominal strength and specimen size. The study revealed change in modes of failure characterized by a jump in brittleness number as loading rate increases from 1 mm/min to 10 mm/min followed by decrease in brittleness number as loading rate increases from 10 mm/ min to 500 mm/min. This conclusion was supported by highly magnified images of failed specimens using scanning electron microscope. A numerical algorithm to determine crack growth resistance curves (R-curves) from peak load is also implemented. The R-curves are geometry as well as loading rate dependent and give G(f), C-f values which agree qualitatively and to some extent quantitatively with the linear regression approach.